This invention relates to transmissions for coupling an engine to the wheel drive line of a vehicle or other driven mechanism and more particularly to transmissions of the type having a torque converter and planetary gear sets for providing a plurality of drive ratios including both fluid and mechanical drive.
Vehicle transmissions enable a broad range of vehicle ground speeds to be realized while the engine is operated within the relatively narrower range of engine speeds at which engine power output is sufficiently high to match the existing load. Thus, subject to practical limitations discussed below, optimum power performance requires that a transmission provide a large number of different drive ratios or transmission speeds. Step ratios between the successive speed ratios should be small and preferably uniform. However, providing for a large number of drive ratios with known techniques tends to add to the bulk, complexity and cost and aggravates certain undesirable performance characteristics. Adding additional gear sets for this purpose increases inertia and friction losses and control is complicated in that more frequent shifting becomes necessary.
Practical transmission designs are usually a compromise between these conflicting considerations. In the interest of simplicity, compactness, economy and ease of control, vehicle transmissions usually provide less than the theoretically optimum number of drive ratios. Further, practical limitations to the relative proportions which may be provided for the gears of a particular transmission design have usually dictated that step ratios between successive drive ratios be somewhat nonuniform and frequently undesirable power reductions are present at one or more specific drive ratios or at transition points between successive drive ratios. These power gaps occur where the gears involved in a particular drive ratio must of necessity provide less than the most efficient ratio match between engine speed and the existing vehicle speed and where the next higher or lower drive ratio is subject to similar limitations at that vehicle speed.
Considering now another transmission characteristic, drive may be purely mechanical through coupled shafts, gears, and the like or may be partly fluid drive by connection of a torque converter into the power path. Mechanical drive has the advantage of being potentially more efficient from the standpoint of power loss but has the disadvantage that repeated shifting of the transmission is necessary to maintain high power throughout if the vehicle speed is changing rapidly or if the vehicle is subjected to a widely varying load. Fluid drive through a torque converter is subject to greater power losses but has a much broader torque range capability. With the torque converter in the power path, the power throughput remains reasonably high for a relatively broad range of vehicle speeds and loads and thus less shifting is needed. Accordingly it is advantageous to provide for selection of either wholly mechanical drive or fluid drive over as broad a range of vehicle speeds as is practical.
Because of the various different desirable performance characteristics and other considerations discussed above, practical transmission designs are always a compromise between conflicting requirements. Prior U.S. Pat. No. 3,347,113 issued to C. A. Ramsel Oct. 17, 1967 discloses a transmission having four forward drive planetary gear sets, an additional planetary gear set for reverse, a torque converter, and which provides highly desirable performance characteristics in terms of the factors discussed above if utilized in certain particular types of vehicle. The transmission of prior U.S. Pat. No. 3,347,113 was designed for use in scrapers of the kind used in earthmoving operations and provides the broad torque capability of fluid drive at a wide range of very low vehicle speeds, at which loading of the scraper occurs, while providing six forward speed ratios of highly efficient mechanical drive at the high ground speeds at which hauling of a load takes place with fairly uniform small step ratios between the six forward mechanical drive speeds. In such a vehicle the broad torque capability at very low speeds is extremely important and is gained at some sacrifice of power transmission efficiency. Power performance at ground speeds slightly above loading speed but below normal hauling speeds is less critical and less than maximum efficiency at these speeds can be accepted. However, this does not apply to many other types of vehicles, such as automotive trucks, wherein a transmission of that general type might be useful. If a transmission of the general type described in prior U.S. Pat. No. 3,347,113 is to be employed in heavy trucks or the like, it is preferable that the available range of efficient mechanical drive be extended to lower vehicle speeds and that converter drive be optional rather than compulsory at intermediate speeds.
While such an extension of the highly efficient mechanical drive range to lower vehicle speeds might be achieved by the addition of more planetary gear sets using known tehcniques, this would involve an undesirable increase in the bulk, complication and cost of the transmission and would increase inertia and friction losses. The problem is further complicated in that it is desirable to avoid imposing increased torque loads on the transmission components as this necessitates an undesirably massive construction for such components. For a given power input, torque loads are an inverse function of speed. Thus a high degree of speed reduction in one part of the gearing system imposes correspondingly high torque loads on the following parts of the system and limit maximum power transmitted.